Recent advances in surface-emitting lasers, or SEL's, have stimulated interest in their potential applications, including monolithic two-dimensional arrays, optical interconnects in very large scale integrated circuits, optical parallel processing and optical neural networks. The implementation of an optical neural network requires two basic components; large arrays of neurons and interconnections therebetween. A neuron may be an optical nonlinear processing element, such as a thresholding device, that can be implemented for example by monolithically integrating a combination of a switching amplifier, a photodetector and a light source such as an SEL on a single chip.
SEL's emit light in a direction normal to the plane of their substrates. Conventional SEL's have been designed in various configurations including horizontal cavity lasers with either internal or external 45.degree. beam deflectors, horizontal cavity lasers with second-order grating couplers and vertical cavity lasers with quarter-wavelength Bragg reflector stacks. Second-order grating lasers and vertical cavity lasers are difficult to monolithically integrate with other optoelectronic devices because of the complexity of fabrication and epitaxial structural incompatibilities.
SEL's with 45.degree. beam deflectors are attractive for use in optoelectronic devices because of the relatively easy integrability and the structural and processing compatibility with other optoelectronic devices. In addition, because of their technological similarity, horizontal cavity SEL's with 45.degree. beam deflectors can take full advantage of the well-developed edge-emitting Fabry-Perot laser technology, but conventional designs typically suffer from relatively high threshold current density and relatively low efficiency when compared with other conventional types of SEL's.
Conventional SEL's with a single 45.degree. beam deflector may be fabricated from a wafer consisting of a GaAs substrate on which double heterostructural layers of AlGaAs and GaAs have been grown epitaxially to form a p-n junction and an optical waveguide. When current is directed across the p-n junction, electrons and holes are injected into and confined within the active layer by the double heterojunction. Light is emitted when the confined electrons and holes recombine in the active layer. The waveguide serves to confine this light to a thin active layer in the plane of the wafer.
Laser cavities, for example, 0.1 .mu.m .times. 4 .mu.m .times. 300 .mu.m, are formed by restricting light laterally, such as by a lateral waveguide in the case of index-guided lasers, or by confining current laterally, as in the case of gain-guided lasers. Vertical laser oscillator mirrors define the longitudinal dimensions of the SEL and are conventionally formed by cleaving and/or etching the wafer crystal perpendicular to the waveguide.
By forming a beam deflector across the light path, 45.degree. to the plane of the wafer, light which would otherwise remain within the waveguide may be reflected away from the SEL in a direction generally normal to the plane of the wafer. The cleaved mirrors themselves, without 45.degree. beam deflectors, allow lasers to emit light only through the edge parallel to the substrate. This limits use of such devices in monolithic integration. Edge emitting lasers of this type are not attractive for use in the implementation of optical neural networks. 45.degree. beam deflectors have been fabricated by several techniques, including chemical etching, ion-milling, ion-beam etching and focused ion beam micromachining.
A horizontal cavity SEL with a 45.degree. beam deflector has been described by Yang et al., in Electron. Lett. 24, 343 (1988). An AlGaAs/GaAs double heterojunction laser is used together with a metalized 45.degree. mirror. One of the two vertical oscillator mirrors is formed by cleaving, while the other is formed by dry etching. The cleaved mirror limits the monolithic integration of this type of SEL with other optoelectronic devices.
While conventional SEL's have met with some success, there remains a need for a more efficient, higher output power and low threshold current density SEL having less power dissipation. The needed SEL's should be compatible with other optoelectronic devices and conventional monolithic IC fabrication techniques for integration in large scale optoelectronic integrated circuits.